060900Z POSITION NEAR 13.5S 110.7E.
TROPICAL CYCLONE (TC) 15S (FIFTEEN), LOCATED APPROXIMATELY 572 NM
NORTH-NORTHWEST OF LEARMONTH, AUSTRALIA, HAS TRACKED SOUTH-
SOUTHWESTWARD AT 04 KNOTS OVER THE PAST SIX HOURS. ANIMATED
MULTISPECTRAL SATELLITE IMAGERY (MSI) SHOWS THE SYSTEM HAS FURTHER
CONSOLIDATED AS CENTRAL CONVECTION DEEPENED, ALBEIT PARTIALLY OFFSET
FROM THE WELL-DEFINED LOW LEVEL CIRCULATION. THE INITIAL POSITION IS
BASED ON THE MSI LOOP AND ON A 060225Z ASCAT BULLSEYE PASS. THE
INITIAL INTENSITY IS BASED ON THE ASCAT PASS AND HELD SLIGHTLY HIGHER
THAN AGENCY DVORAK ESTIMATES OF 30 KNOTS. UPPER LEVEL ANALYSIS
INDICATES THE SYSTEM IS IN AN AREA OF STRONG (25-30 KNOT) WESTERLY
VERTICAL WIND SHEAR (VWS). ADDITIONALLY, A MID-LATITUDE TROUGH IS
CAUSING SUBSIDENCE ALONG THE SOUTHWESTERN QUADRANT. HOWEVER, THE VWS
AND SUBSIDENCE ARE MOSTLY OFFSET BY A STRONG POLEWARD OUTFLOW THAT IS
PROVIDING EXCELLENT VENTILATION TO THE CONVECTION. TC 15S IS TRACKING
ALONG THE NORTHWEST PERIPHERY OF A SUBTROPICAL RIDGE (STR) TO THE
SOUTHEAST. IT IS EXPECTED TO CONTINUE TRACKING SOUTH-SOUTHWESTWARD
THEN ON A MORE WESTWARD TRACK AS THE STR BUILDS. THE STRONG OUTFLOW
WILL PROMOTE MODEST INTENSIFICATION OVER THE NEXT 24 HOURS.
AFTERWARDS, INCREASING VWS WILL PREVAIL AND GRADUALLY ERODE THEN
EVENTUALLY DISSIPATE THE SYSTEM BY TAU 72. AVAILABLE NUMERIC GUIDANCE
ARE IN OVERALL AGREEMENT WITH THE FORECAST TRACK; HOWEVER, THERE IS A
SIGNIFICANT SPREAD IN THE TRAJECTORY, LENDING LOW CONFIDENCE TO THE
JTWC INITIAL TRACK FORECAST. MAXIMUM SIGNIFICANT WAVE HEIGHT AT
060600Z IS 10 FEET. THIS CANCELS AND SUPERSEDES REF A (WTXS21 PGTW
060130). NEXT WARNINGS AT 062100Z AND 070900Z.//
NNNN
IDW27600
TROPICAL CYCLONE TECHNICAL BULLETIN: AUSTRALIA - WESTERN REGION
Issued by PERTH TROPICAL CYCLONE WARNING CENTRE
at: 0700 UTC 06/04/2017
Name: Tropical Low
Identifier: 26U
Data At: 0600 UTC
Latitude: 13.4S
Longitude: 111.0E
Location Accuracy: within 40 nm [75 km]
Movement Towards: south southwest [196 deg]
Speed of Movement: 5 knots [10 km/h]
Maximum 10-Minute Wind: 35 knots [65 km/h]
Maximum 3-Second Wind Gust: 50 knots [95 km/h]
Central Pressure: 1000 hPa
Radius of 34-knot winds NE quadrant:
Radius of 34-knot winds SE quadrant:
Radius of 34-knot winds SW quadrant: 60 nm [110 km]
Radius of 34-knot winds NW quadrant: 60 nm [110 km]
Radius of 48-knot winds NE quadrant:
Radius of 48-knot winds SE quadrant:
Radius of 48-knot winds SW quadrant:
Radius of 48-knot winds NW quadrant:
Radius of 64-knot winds:
Radius of Maximum Winds:
Dvorak Intensity Code: T2.5/2.5/D1.5/24HRS STT:S0.0/06HRS
Pressure of outermost isobar: 1006 hPa
Radius of outermost closed isobar: 120 nm [220 km]
FORECAST DATA
Date/Time : Location : Loc. Accuracy: Max Wind : Central Pressure
[UTC] : degrees : nm [km]: knots[km/h]: hPa
+06: 06/1200: 13.7S 110.9E: 050 [095]: 035 [065]: 998
+12: 06/1800: 14.2S 110.9E: 065 [120]: 035 [065]: 998
+18: 07/0000: 14.6S 111.2E: 075 [140]: 040 [075]: 995
+24: 07/0600: 14.9S 111.4E: 090 [165]: 040 [075]: 995
+36: 07/1800: 15.4S 111.8E: 110 [200]: 050 [095]: 988
+48: 08/0600: 15.9S 111.4E: 130 [235]: 045 [085]: 992
+60: 08/1800: 16.3S 110.5E: 150 [275]: 045 [085]: 992
+72: 09/0600: 16.8S 109.0E: 165 [310]: 040 [075]: 993
+96: 10/0600: 18.0S 105.1E: 210 [390]: 035 [065]: 1006
+120: 11/0600: 19.6S 100.8E: 300 [555]: 030 [055]: 1006
REMARKS:
Tropical low [26U] was located using visible and microwave imagery together with
recent ASCAT passes.
26U has developed considerably overnight. Initial T1 classification was assigned
at 05/0600 UTC.
Dvorak Analysis: Curved band wrap of between 0.4 and 0.5 [with some breaks in
convection] gives a DT of 2.5 [a shear pattern would yield DTs of 3.0]. Trend
was D+ and MET/PAT was 2.5. FT/CI set to 2.5. Intensity set to 35 knots with
gales in the NW and SW quadrants. Most recent visible imagery showed a slight
weakening in the structure of the system.
ASCAT passes around 02 UTC showed gales in the NW and SW quadrants. This is
consistent with microwave imagery earlier in the day with persistent deep
convection in those quadrants.
SSTs are 29-30C and ocean heat content is favourable.
CIMSS showed good poleward outflow and upper divergence. CIMSS shear of 15/20
knots appears a little higher than satellite imagery would suggest.
The system is forecast to become a tropical cyclone during Friday. Given the
lack of strong synoptic forcing, the intensity may fluctuate somewhat.
Intensification to category 2 is possible for a period. The presence of dry air
and increasing shear late Sunday should cause the system to weaken, however,
gales may persist in southern quadrants due to the pressure gradient associated
with a ridge of high pressure to the south.
The system is being steered towards the south southeast due to an upper trough
passing to the south. On Saturday, a building mid level ridge will steer the
system towards the southwest. The majority of NWP guidance is consistent with
the forecast track.
Tropical low [26U] is not expected to produce gales at Christmas Island or the
WA mainland.
